Boo Radley was locked in the house as a teenager because he was unstable and involved with a group of troublemakers and the family did not want him to go to jail.

The Radley House is “three doors to the south” to the Finches and the children avoid it because it is “inhabited by an unknown entity,” the mysterious “malevolent phantom” Boo Radley (ch 1). When Scout and Jem were children, the Radleys kept to themselves and rarely left the house.

According to neighborhood legend, when the younger Radley boy was in his teens he became acquainted with [a group of boys that] formed the nearest thing to a gang ever seen in Maycomb. (ch 1)

The boys did not do much more than hang out, but they were the talk of the town. One night they harassed a beadle and were arrested and sent to “state industrial school.” Mr. Radley thought it was a disgrace and asked that Arthur be released to his custody, and “Mr. Radley's boy was not seen again for fifteen years.”

Then when day when he was 33 years old, Boo Radley stabbed his father in the leg with scissors. He was arrested, sent to jail, and once again released to the Radley’s custody—and never seen again.

Boo Radley’s story is a tragic one. By all accounts, he was not a bad kid. He simply was bored. His mental problems were more likely to result of his shamed and reclusive religious zealot father. By the time he gets to know Scout and Jem, he is just a shy, timid, shell of a man who is terribly lonely but has a good heart.